142 ORVILLE A. DERB\ 



The possibility of an admixture of originally eruptive elements 

 in the phyllites themselves is, as already noted, suggested by the 

 supposed copper staining of Burton and also by the harder olive- 

 green clay that he mentions as occurring with the second body. 

 The only rock specimen that has come to hand from the mine is 

 a small fragment of a sericitic schist that, aside from a very fine 

 dust of hematite, gives no residue whatever, and which may be 

 suspected to be a metamorphosed eruptive. 



On the hypothesis of the original essentially pegmatitic 

 character of the diamond-bearing bodies of Sao Joao da Chapada 

 three important questions arise which can only be solved hypo- 

 thetically. What was the original type of the pegmatite ? Is 

 it eruptive or secretionary ? Do the diamonds belong to it or to 

 the country rock in its immediate vicinity, and perhaps modified 

 by it, or to both ? 



Bodies of pegmatite are quite common in the older rocks of 

 Brazil, both in the diamond regions and elsewhere, occurring 

 not only in the gneiss and granite, but in the schistose series as 

 well. Those that have been examined are dike-like in their mode 

 of occurrence and granitic in composition. They are almost 

 universally decomposed, affording a pulverulent kaolin, not the 

 indurated type of lithomarge. Their residues are usually abun- 

 dant and typically granitic, representing more particularly the 

 type of the muscovite granites, consisting of zircon, monazite, 

 and almost invariably xenotime. All of these characteristics 

 (which, however, may not be essential) are lacking in the sup- 

 posed pegmatitic clay of Sao Joao da Chapada, in which only 

 the presence of quartz is suggestive of granite affiliation. On 

 the other hand, however, they are compared with great propriety 

 by Gorceix with the topaz-bearing clays of Ouro Preto, and 

 topaz is generally regarded as a granitic mineral. Topaz has 

 not been reported from Sao Joao da Chapada, but in one wash- 

 ing from a mixed sample of the clays a minute grain was 

 observed that in form, optical properties, and specific gravity 

 seems to agree with that mineral. The other known types of 

 pegmatite — those affiliated with syenites, diorites, and gabbros 



